Following virulent Mycobacterium tuberculosis (Mtb) infection in mice, a robust T cell response is primed in the draining LN, undergoes massive expansion, and traffics to the lung. Upon reaching the lung, T cells abort exponential bacterial growth, which leads to an early decline in bacterial burden followed by stabilization of the bacterial numbers long term. Despite T cell mediated clearance of most bacteria, sterilization is never achieved. Under optimal conditions, the best vaccines provide a 20-30-fold reduction in lung CFU. Thus, an optimal T cell response clears many but not all of the infecting bacteria-suggesting an important hurdle to achieving better protective immunity to infection is to determine why a relatively small, but biologically important subpopulation of bacteria survive in the face of otherwise effective T cell mediated immunity. Importantly, while the quantitative balance between successful immunity and failure may vary between animal models and people, both have features of both immune control and escape. Mice clear most bacteria after the onset of adaptive immunity but are unable to sterilize the lung, even if vaccinated. Thus, murine TB may be a reasonable model to explore why T cell immunity fails. Why might a subpopulation of bacteria survive in the face of a T cell response that can clear most of the bacterial population? There appears to be something different about the infectious course of the bacteria that survive in the face of robust adaptive immunity as compared to the ones that are cleared. Heterogeneity may arise at the level of the bacterium, the cellular compartment in which it resides, or the T cells that encounter infected cells. Understanding the balanced success and failure of T cell immunity to Mtb is important for determining how to better design a vaccine against Mtb infection. It is currently not clear whether making a new subunit vaccine with a different combination of antigens; application of a new adjuvant or a different attenuated strain of Mtb will solve this problem. We postulate that the first step towards designing a vaccine that elicits immunity that is better than that elicited by natural infection or current vaccines is identifying he features that drive immune failure. Our coordinated effort will investigate three fundamental questions about T cell immunity to Mtb, with the goal of having a major impact on vaccine development. Our overarching hypothesis is that bacteria survive despite a robust T cell response because of a local failure in T cell surveillance and effector function. Aim 1. Is T cell recognition of a subpopulation of infected cells impaired? We hypothesize that T cells recognize many but not all infected cells, and promote Mtb clearance. What remains is a population of infected cells that cannot be recognized or activated by T cells, and provides a niche for Mtb persistence. We will identify and determine why this niche emerges. Aim 2. Do quantitative differences in the bacterial population allow some Mtb cells to escape T cell clearance? We hypothesize that some infected cells escape recognition because expression of the early antigens that primed the immune response are downregulated later in infection. We will use bacterial strains that have been engineered to allow rheostat-like control of antigen production to determine whether quantitative differences in antigen load alter T cell recognition or effector function. Aim 3. Do distinct cell types process and present Mtb antigens differently? The T cell response reflects the antigens processed and presented by the priming DC. However, during HIV infection, intracellular processing of proteins and thus antigen presentation varies in different cell types (macrophages vs. DC) and with different activation states. We will test this hypothesis for Mtb infection, postulating that differences in the peptide epitopes presented by different cell types allow some infected cells to escape T cell surveillance.